The University of Oklahoma Section of Reproductive Endocrinology and Infertility is uniquely positioned to provide a geographically distinct and ethnically diverse study site for the Reproductive Medicine Network. The OU-RMNU brings a cadre of clinician scientists and physicians to the RMN as well as a research nurse, a regulatory specialist and a database manager. This experienced team has been involved in clinical trials and investigations involving unexplained infertility, polycystic ovarian syndrome, and reproductive aging in women. In addition, our strengths include our large clinical volume and the wide variety of reproductive disorders treated in our clinic. Our concept proposal titled Luteal Progesterone Supplementation in Clomiphene Citrate-lntrauterine Insemination (CC-IUI) Treatment Cycles for Unexplained Infertility provides a demonstration of the collaborative nature of our section and our research interests. Recent observational and small prospective studies have suggested that luteal phase progesterone supplementation may improve live-birth rates in women with unexplained infertility undergoing superovulation-IUI treatments. However, all of these investigations have methodological weaknesses and are underpowered. Given the low cost and high utilization of these treatments as compared to the cost of IVF, even small improvements in live-birth rates would be clinically meaningful. The primary goal of this investigation is to determine whether progesterone supplementation in the luteal phase improves the live-birth rate in couples undergoing treatment with CC-IUI for unexplained infertility. Secondary aims include evaluating differences in clinical pregnancy and spontaneous abortion rates in women treated with progesterone supplementation compared to women in the placebo control. This investigation will address the above questions with an appropriately powered multicenter, placebo-controlled, double-blinded clinical trial that will enroll 720 couples with unexplained infertility. The identification of effective evidence-based treatments for couples with unexplained infertility is vitally important given the burden of infertility on society, and i directly responsive to the NICHD's mission.